1. Field of the Invention
This invention relates to improved gauge apparatus for measuring and indicating the quantity of a pressurized gas within a confining zone based upon sensings of pressure and temperature. More particularly, such apparatus is concerned with employing electrical techniques for continuously and automatically compensating sensings of the pressure of the gas in accordance with the temperature of the gas to provide an accurate on-going indication of the quantity of the gas currently within the zone of interest, even in applications in which gas is being withdrawn from the mentioned zone. By virtue of its attributes of simplicity, reliability, ruggedness and relatively low cost, the invention has special immediate applicability for providing fuel gauges suitable for use in vehicles powered from compressed natural gas or other pressurized gaseous fuels, although it will be understood that the invention has wider potential applicability in virtually any situation requiring the gauging of the quantity of a pressurized gas within a given zone or system.
2. Description of the Prior Art
It has, of course, long been known from the Ideal Gas Law of elementary physics that the quantity (e.g., number of moles) of an "ideal" gas confined within a given volume is proportional to the pressure of the confined gas divided by its temperature multiplied by a constant (e.g., the "ideal gas constant") consistent with the assumption of an "ideal" gas and the units of measure chosen (e.g., atmospheres for pressure and degrees Kelvin for temperature). It has also been heretofore recognized that, although the general relationship represented by such law still applies in a qualitative sense (and, frequently, to a very close approximation in a quantitative sense) when real gases are involved, the applicable constant of proportionality will be influenced by specific gravity or density of the particular gas involved and, perhaps, other factors. it is also known, therefore, that the actual relationship between the quantity, the pressure and the temperature of a particular confined gas may need to be determined or confirmed empirically for greatest accuracy of subsequent measurement, even though such relationship will typically be found to follow the Ideal Gas Law to a surprisingly close approximation.
It is also known that various approaches to applying some form of compensation for temperature to measurements of the pressure of a gas have been proposed. A first group of previously proposed arrangements for such purpose of which I am aware, in which the compensation for temperature is sought to be effected through primarily "mechanical" means, are the Posnansky U.S. Pat. No. 3,934,479 (altering the volume of a pressure sensing chamber), the Olsen U.S. Pat. No. 4,198,867 (altering the tension of a vibrating wire), the Miyamae U.S. Pat. No. 4,206,655 (altering the effective dimension of a base supporting pressure sensing components), and the Bleidt et al U.S. Pat. No. 4,214,474 (altering the condition of a bimetallic element). A second group of previously proposed arrangements relating to some form of temperature compensation in connection with the measurement of the pressure of a gas of which I am aware, in which the compensation for temperature is sought to be effected through primarily electrical or electro-optical means, are the Reesby et al U.S. Pat. No. 3,301,062 (employing a light source, a mirror, a pair of photocells, a manually rotatable support for the latter, a helical type fused quartz tube pressure sensor for rotating the mirror relative to the light source and the photocells, a potentiometer for electrically representing the position to which the support for the photocells is manually rotated, a nul meter and an electrical circuit including a temperature sensitive thermistor), the Brandau et al U.S. Pat. No. 3,528,293 (employing an electrical circuit requiring multiple reference voltage inputs, a thermistor and a servosystem), the Pearson U.S. Pat. No. 4,000,643 (employing operational amplifiers and a resistance network), and the Waugh U.S. Pat. No. 4,226,125 (employing piezoresistors, electronic gate circuitry and timed sampling techniques). The last mentioned group of patents involve proposed arrangements that may be generally characterized by their relative complexity and apparent better suitability for implementation in a laboratory type environment than in practical applications in the field such as primarily contemplated by the present invention.
Although no claim is made herein with respect to any individual electrical component per se, the existence of such components as negative temperature coefficient type thermistors and potentiometers in electrical measuring or gauging circuits should, perhaps, be here confirmed as of itself relatively common in a general sense, for example in the Sias et al U.S. Pat. No. 2,409,073 (involving an electrical circuit arrangement for the gauging of a liquid fuel supply in which a negative temperature coefficient resistance was utilized in connection with an oscillator for compensating for the effects of temperature upon the frequency representative of the measurement made), and the Payne U.S. Pat. No. 2,614,422 (involving an electrical arrangement employing variable resistances for measuring the rate of consumption of a liquid fuel and estimating the time duration before exhaustion of the latter).
Thus, although I am aware of the conventionality of certain of the individual components employed in implementing the present invention, which is one of its practical virtues, I am not aware of anyone having previously combined such components in a similar circuit arrangement for similar purposes or with the accomplishment of similar results. Indeed, the practical state of the prior art is thought to be essentially that, although more elaborate and expensive types of equipment particularly suited for use in a laboratory environment may have been available for at least theoretically accomplishing temperature compensated measurements of pressure of a confined gas, factors such as the very complexity of such equipment, its relatively high costs and/or its inherent lack of ruggedness and reliability under field conditions has heretofore essentially brought about a situation in which those concerned with providing some form of gauging for tank carried gasses in adverse environments, such as in systems for powering vehicles with compressed natural gas or similar fuels, have apparently concluded that any attempt to compensate for the effects of temperature in such installations would be impractical and have, therefore, elected to provide merely a measurement and indication of the pressure of the confined fuel as a rough approximation of the pressurized gas fuel supply that may remain.
It is upon that background of previous absence of a practical means for providing accurate gauging of the quantity of a pressurized gaseous fuel supply remaining available on a vehicle or the like that the present invention has been developed to provide a solution to the problem suited by its nature and characteristics for widespread usage in appropriate applications, such as the increasing number of vehicles that are expected to be powered in the future from compressed natural gas or similar fuels because of economic and other energy related considerations.